It's hard to find a decisive answer (I tried). The choice I made here was a matter of taste, and it was based on the sentence context.

When you grasp the edge of a blanket - samekh is more common and that would be my first choice.

When you grasp an idea - you can still use samekh and it's perfectly fine, but you can also use shin, which is a bit more subtle (to my opinion)
and it helps differentiate between the two. It might has something to do with the archaic language. I think samekh has become more popular these days.